Among a number of factors defining the quality of a road with an asphalt pavement, the main ones are the compactness and the strength of the latter or, more exactly, of the structural layers made therefrom. If the strength of the pavement is chiefly defined by a number of processing factors implemented in course of the asphalt concrete preparation, the pavement compactness results only from the treatment of the already finished paving asphalt mixture directly on the road, that is from the mechanical compaction process. The aim of the mechanical compaction is, as a rule, to produce a material with a low degree of porosity that is a basic characteristic defining, together with the physical strength of the road pavement material, its endurance in service. If the porosity exceeds the tolerable limits, this will lead to a quick destruction of the pavements under the conditions of sharply varying climatic conditions, especially in northern latitudes, and will require additional expenditures for its repair. The cause of pavement destruction is moisture getting into pores in the pavement, that changes its volume depending upon the temperature and, thus, produces, except the service loads, additional internal loads on the pavement, stresses that are often practically the only cause of its destruction.
As soon as the minimum pavement porosity is achieved as well as its maximum strength depending, to a certain degree also upon the porosity, one tries to select such compaction conditions that stresses produced by compaction machines in the paving asphalt mixture laid into the road basement be in a certain relationship with the ultimate strength of this mixture. This relationship is .tau.--0.94-0.98.tau..sub.us, where .tau.--current value of stress produced in the paving mixture and .tau..sub.us --ultimate strength of the paving mixture.
Another very important factor for which allowance is made in the paving mixture compaction is that the mixture is laid onto the road base or bed at a temperature of 140.degree. to 160.degree. C., but this temperature decreases down to the ambient temperature in course of compaction. The temperature decrease is exponential. In the light of the aforesaid the compaction process appears to be very labour-consuming because .tau..sub.us varies its value with temperature, and this means that .tau. should be continuously changed following the variation of .tau..sub.us in accordance with the above relationship. In other words, the load applied to the paving mixture in course of its compaction should be also varied exponentially, increasing continuously. Reasonable paving asphalt mixture temperatures within which the compaction is accomplished are from 130.degree. C.-150.degree. C. to 70.degree. C.-60.degree. C.
When modern compaction rollers are being developed for pavement made from hot or warm asphalt, one tries to meet to a great extent the requirement to increase gradually the load acting on the cooling mixture. In this connection, a method of pavement compaction is presently adapted that is based on the successive use of several rollers of different weights, from light rollers to heavy ones. Due to this fact it is customary to divide all rollers into three categories depending upon their mass: light--with a mass up to 5T, medium--with a mass from 5 to 10T, and heavy--with a mass over 10T. Although such a division is universally accepted, it is nevertheless incomplete: a deeper mass differentiation is possible within every weight category. So, for example, the light category includes rollers with a mass of 1.5T, 3T, 5T.
In order to implement the above mentioned method it is necessary to have from 3 to 7 and more rollers of various weights. It should be noted that there are already being developed compaction rollers implementing the exponential law of load variation during the compaction, however these rollers at present are practically no more than subjects of laboratory investigations.
The modern asphalt pavement compaction process is simple in itself, but requires the time intervals of operation of rollers of every weight category to be maintained with a high accuracy. For this purpose, the pavement temperature is measured continuously in order to replace the roller of one weight by another one that is more heavy, in due time, to compact the pavement. Really, the pavement temperature is the only objective criterium that can be used in course of compaction of the pavement since the attempt to measure .sigma..sub.us is merely impracticable under field conditions, although this parameter allows to judge more objectively the necessity to replace one type of roller by another than the temperature. Nevertheless, the measurement of the latter is most widely used due to its practicability.
Various methods and devices are known allowing determination of the pavement temperature in course of compaction. The contact methods and devices of the type described in the USSR Author's Certificate No. 52,094 is the most simple among them. This method is based on the contact measurement of the heated body surface temperature. The device implementing this method is a thermocouple made as an elastic conical helical spiral having its base secured to a thrust ring.
These method and device are very simple. However, their use results in a considerable error in temperature measurement because of heat exchange between the thermocouple and the environment as well as because of thermal sluggishness. The principal problems are purely technical and are caused first of all by the bitumen sticking to the surface of the sensing member--the thermocouple. Any measures aimed at elimination of this disadvantage would lead to a reduction in the measurement accuracy and to an increase in the radiation heat exchange as well as in the sluggishness. Furthermore, the contact theremometer responds only to the surface temperature that is mostly not equal to the actual, so-called average layer cross-section temperature, i.e. the temperature that shall be taken into account in the asphalt compaction process.
Also known in the prior art are a contact method of soil or asphalt temperature measurement and a device for its implementation that are partially free from the disadvantages inherent in the method and the device discussed hereinabove (cf. USSR Author's Certificate No. 77,795). According to this method the sensing member of the instrument is immersed into the soil or asphalt layer to measure its temperature. The device used to implement this method is a cylinder filled with liquid and provided with a graduated tube communicating with the cylinder and filled with the same liquid. The tube shows the variation of the liquid volume in the cylinder upon heat expansion of the liquid and gives indirect information about the temperature of the soil or asphalt layer into which the cylinder has been immersed.
These method and device are intended for measuring just the average soil or asphalt temperature. However their use requires to immerse the cylinder into the pavement each time the temperature is to be measured, thus disturbing its continuity, to say nothing of the considerable labour required for the immersion itself which should be repeated tens of times during one compaction cycle of 40 to 60 minutes long.
Also known are non-contact methods of measuring the surface temperature of bodies, in particular of the asphalt mixture. Among these most widely used are pyrometric methods. The devices implementing these methods are called pyrometers. In the pyrometric method of temperature measurement the heat radiation of heated objects is measured. Depending upon the nature of information derived the pyrometers can be devided into devices for local measurement of temperature in a given point of the object and devices intended to analyse temperature fields (thermovisors). Depending upon the principle of operation one distinguishes luminous filament, colour and radiation pyrometers. The luminous filament, visual pyrometers are used for measuring luminance temperatures over 600.degree. C. Their principle of operation is based on the dependence of the spectral luminance of the heated bodies on their temperature, described by the planck and Wien laws. The action of the colour pyrometers is based on the comparison of object radiation intenssities in two spectral ranges. The logarithm of their ratio varies inversly with the object colour temperature.
The action of the radiation pyrometers is based on the use of the Stefan-Boltzmann law expressing the relationship between the energy radiated by a body and its temperature. The instruments of this type are widely used for measuring low temperatures (20.degree. to 100.degree. C.), thermocouples and bolometers serving as radiation receivers. Thermopiles (thermocouply connected in series) are frequently used.
Known in particular is a pyrometer used as a rolling monitoring device allowing the temperature of asphalt pavement surface to be measured continuously and remotely (see the book V. V. Badalov et al. "Effect of process and mechanization means on quality of asphalt concrete road pavement production", Leningrad House of Technology (LDNTP), 1977, p. 21). The device is installed on a roller and comprises two units: indicating and measuring. The measuring unit of the device is fastened to the roller frame at a distance of 80 to 100 cm from the asphalt pavement surface. The indicating unit is arranged on the instrument board of the operator's control station. The device is operated from the roller storage battery.
It should be noted that the non-contact methods of temperature measurement described hereinabove and pyrometers used for this purpose and, in particular, the rolling monitoring device are rather complicated and are very expensive. Furthermore, under all circumstances they measure only the surface temperature of bodies while under the working conditions the asphalt surface temperature differs materially from the average layer temperature of from the temperature inside the asphalt layer. In course of compaction the surface of the asphalt layer is always covered with water spots that appear due to the compulsory abudant wetting of roller drums with water. This is done to prevent asphalt sticking to the roller drums. The surface water disturbs the temperature measurement. One is forced either to introduce large corrections into the measurement results or to find surfaces that are not covered with water.
One of the non-contact methods used presently for measuring the object temperature is the method comprising the measurement of temperature of gaseous heat-carrying agent used to blow the surface of this object. Such a method and respective device are described in USSR Author's Certificate No. 251,870.
When applied to the measurement of asphalt pavement temperature the abovementioned method and device possess a number of disadvantages the main of which are insufficient accuracy and reliability. This is caused by the strong effect of the surface condition of the asphalt pavement that is abudantly wetted due to the reasons mentioned hereinabove as well as by the atmospheric conditions, i.e. by the wind and the air temperature. All this requires that the temperature readings determined with the aid of this method be corrected, but the determination of these corrections is difficult.
The method and device adopted as a prototype are to a certain degree free from the disadvantages inherent individually in each method and device used for radiation temperature measurement and its measurement by means of a gaseous heat-carrying agent. This method and the device used for implementing the same are described in USSR Author's Certificate No. 51,477. The essence of this method consists in particular in that the surface temperature of the soil and other similar materials is determined from the temperature of a heat-conducting casing accommodating a radiation energy indicator and from that of the air enclosed within the casing at the instant the radiation heat exchange balance is established between the surface being measured and the parts of the measuring device. At junctions of the radiation energy indicator, for example a thermopile, included into a certain system with other bodies the radiation energy balance should be zero if all bodies of the system have the same temperature. This follows directly from the Kirchhoff's law. The method is implemented by a device comprising a metallic massive casing open on one side and provided with a heating winding of high-resistance wire. The casing is covered with thermal insulation, and a mercurial thermometer is arranged therein. Accommodated inside the casing is a radiation energy indicator sensing the radiation of the surface being investigated. This indicator is connected electrically with a galvanometer and closed with a special window made from rock-salt or fluorite whereas the casing is silver plated inside.
The method and the device mentioned hereinabove increase slightly the temperature measurement accuracy as compared with those discussed above since in this event the effect of environmental, in particular atmospheric factors is avoided.
However the accuracy and reliability of temperature measurements with the use of this method do not meet the requirements of the modern asphlat compaction process.
Really, the moisture on the pavement surface will materially change the radiation. In other words, the radiation intensity will continuously vary depending upon the variation of pavement wetness the constancy of which can be practically never guaranteed. Furthermore, said method and device, give no idea about the temperature inside the pavement layer, but this is that in which the technologists performing the compaction are interested.
Thus, the confidence of the obtained temperature measurement rasults will be low. Worthy of notice is also the complexity of the device itself which comprises such capricious components as galvanometer, controlled heating winding and radiation energy indicator.